MDL-87341 javascript: Fix aria label issue for navigation bar

lib/amd/src/paged_content_paging_bar.js

@@ -408,8 +408,11 @@ function(
         var pageItems = root.find(SELECTORS.PAGE_ITEM);
         // We want to request all of the strings at once rather than
         // one at a time.
-        var stringRequests = pageItems.toArray().map(function(index, page) {
+        var stringRequests = pageItems.toArray().map(function(page) {
             page = $(page);
+            if (page.attr('data-page') == undefined) {
+                return {};
+            }
             var pageNumber = getPageNumber(root, page);
 
             if (pageNumber === activePageNumber) {
@@ -425,7 +428,7 @@ function(
                     param: pageNumber
                 };
             }
-        });
+        }).filter(Boolean);
 
         Str.get_strings(stringRequests).then(function(strings) {
             pageItems.each(function(index, page) {